"After I thought about it awhile, I actually felt an obligation to give it some genuine thought," he told the Missoulian on Monday. "I had an obligation to progressive policy, to Montana and to the institution of Congress, which I still respect, even though the Congress has fallen on hard times."
Williams's decision makes him the latest in a string of Democrats to turn down a bid for the open seat after former Montana Gov. Brian Schweitzer (D) decided against a bid, giving Republicans a prime pickup opportunity when Sen. Max Baucus (D-Mont.) retires.
Democrats who haven't ruled out a run for the seat include recently elected Lt. Gov. John Walsh and Montana state Supreme Court Justice Brian Morris.
Many Republicans expect Rep. Steve Daines (R-Mont.) to run for the seat.